1. Field of the Invention
The present invention relates to a code division multiplex (CDM) transmitting and receiving system, and in particular to a CDM transmitting and receiving system for communication through a passive optical network. The invention also relates to a CDM signal receiver.
2. Description of the Related Art
There is great interest in passive optical networks (PONs) as a means of providing bidirectional optical communication between a network operator and a plurality of subscribers. A PON has a star network topology centered on a passive optical coupler (star coupler) that couples a single optical transmission path leading to the network operator's transmitting and receiving apparatus to a plurality of optical transmission paths leading to subscribers' transmitting and receiving units. In PON terminology, the network operator's signal transmitting and receiving apparatus is called an optical line terminal (OLT), and the subscribers' signal transmitting and receiving units are called optical network units (ONUs). The main advantage of a PON system is that by sharing the optical transmission path to the OLT among a plurality of subscribers, it can reduce system equipment costs. General descriptions of PON systems can be found in Ohnishi et al., ‘Isanetto PON shisutem’ (Ethernet PON system), Fujikura Giho, No. 102, April 2002, pp. 18-21, and Ashi et al., ‘PON Based All Fiber-Optic Access System for High-speed Multimedia Services’, Hitachi Review, Vol. 48, No. 4, 1999, pp. 229-233.
A PON system may also have a cascaded star network topology, with two or more star couplers. When ONUs are referred to in the following description, it will be understood that they are a group of ONUs connected through one or more passive optical couplers to a single OLT. Communication from the OLT to the ONUs will be referred to as downstream communication, and communication from the ONUs to the OLT as upstream communication. Communication between the OLT and N ONUs can also be described as 1-to-N communication, where N is the number of ONUs.
The conventional optical access network systems described by Ohnishi et al. and Ashi et al. employ time division multiplexing (TDM), and separate signals transmitted from different subscribers by assigning different time slots to the subscribers. PON optical access network systems may also employ code division multiplexing (CDM), with or without TDM. A PON system using CDM will also be referred to below as a CDM-PON system.
In CDM communication, the transmitter encodes and transmits a signal, and the receiver decodes the received signal by using the same code as in the transmitter. One of the advantages of CDM transmission is its inherent security, in that all communication is coded with a key shared by the transmitter and receiver (the code itself is the key). Another advantage is that multiplexed signals are transmitted simultaneously, enabling a large amount of data to be transferred while conserving communication resources such as wavelengths and time slots. Tamai et al. describe an exemplary CDM-PON system in Japanese Patent Application Publication No. 2007-228134 and disclose a ranging method by which signal timing is adjusted to compensate for the different distances between the OLT and the individual ONUs.
A CDM-PON communication system requires clock signals for ranging, decoding, gating, and other operations. TDM transmission and receiving systems generally extract clock signals from TDM signals by detecting the modulation frequency of the TDM signal. The clock signal is extracted by a device termed a clock data recovery (CDR) device.
A TDM signal, however, is a bi-level digital signal, whereas a CDM signal is a multi-level signal with more than two levels. Conventional CDR devices are designed to operate with bi-level signals and cannot recover clock signals from CDM signals.
Although CDR generally denotes the recovery of both clock and data signals from a transmitted signal, the term ‘CDR’ as used herein refers only to the recovery of the clock signal, and a CDR device will also be referred to as a clock recovery device.